The present invention relates to low noise electronic circuitry and more particularly to such circuitry for use with transducers.
All electronic circuit components internally modulate electrical signals producing internally generated noise due to some finite value of random internal electronic motion. This characteristic is more pronounced in active circuit components and places a minimum noise level on signals emanating from circuits using semi-conductor elements. Additional difficulty has been experienced in the past with circuitry provided in electro-mechanical transducers, such as the circuitry disclosed in U.S. Pat. No. 3,109,145. Such a circuit requires feedback means for acceptable operation because the active circuit components have widely varying gain characteristics caused by changes in ambient temperature and physical variation within a component group. The active components have limiting signal levels to which they may be exposed and beyond which they suffer permanent structural damage. The resulting functional variations can only be controlled within the narrow operating limitations by providing feedback within input circuit to closely control the amount of drive at the inpu to maintain normal operating levels within the circuit.
Many circuits operating in conjunction with electro-mechanical transducers provide a signal proportional to the transduced quantity which is detected and amplified by a high gain amplifier. A minute amount of noise modulation by the active circuit components is amplified by a large factor; for example, 1,000. Amplifier output as a consequence provides an erroneous indication of the quantity being monitored.
Whenever requirements arise calling for the conversion of the deflection of a moving system into an electrical output with an accompanying wide band width of flat response, the physical displacement must be by design small, so that the natural frequency of the mechanical system will be high. In such cases, the noise level of the transducer and its electronics becomes a limiting factor in system performance. Obviously a need exists for a low noise electronic circuit independent of such internal noise modulation for use with transducers of the above type.